I taught an undergraduate class titled Climate Change Science this Spring. Without all the gory detail, below are six lessons that I’ve distilled from the class material.
1. Climate is fundamentally different than weather.
One of the biggest mistakes that people make when talking about climate change is mistaking the weather and the climate. We’ve all seen social media posts (or newspaper articles!) connecting unseasonably hot days to climate change or exploiting unusually cold snaps to suggest that climate change is not real. Another misconception arises when people ask how we can predict the climate by the end of the century if we can’t predict the weather two weeks ahead?
Weather is famously chaotic, as demonstrated by the ‘Butterfly Effect’ – the idea that a butterfly flapping its wings in Brazil could cause a tornado in Texas. In other words, very small things that we’ll never know can have huge consequences for the weather in the future. For this reason, we can’t predict the weather more than two weeks out. The small details start to matter too much! However, the climate isn’t chaotic. As an example, I can tell you with extremely high confidence that the average temperature in Texas this August will be greater than it was in January! I can also tell you with high confidence that it will be miserable in Texas in August. Just because we can’t predict the weather more than two weeks out doesn’t in principle mean that we can’t predict the climate 100 years ahead. Don’t get me wrong, predicting the climate that far ahead is hard as I describe below, but the reasons why are fundamentally different from the reasons why predicting the weather is hard.
The climate is the statistics of the weather. A famous example of something that is unpredictable is flipping a coin. You can’t predict an individual outcome – it could be heads or tails. But the statistics are predictable! You can predict that with 1000 flips, roughly 500 will be heads and 500 will be tails. A single weather event is like a single coin toss. We can’t attribute it to a statistical thing like the climate. But we can in principle predict the statistical properties of something, even if we can’t predict an individual outcome.
2. Earth’s climate has never been fixed.
The term ‘climate change’ is a bit of a misnomer. Most people use the term to describe the warming trend that has followed the Industrial Revolution, but that implies a change on top of something fixed. In fact, change is baked into the fabric of our planet; it just usually happens slow enough that we’re unaware of it. We also happen to be living in a period of relative climate stability that we call the Holocene. Humans living before the Holocene, in the Pleistocene, would have experienced much more noticeable climate changes.
Some of the initial evidence our modern society gathered that our climate wasn’t always the same comes from observations of the landscape in northern Europe and North America, which were covered by glaciers around 21,000 years ago. This evidence is recorded in grooves carved in rock, big boulders that got dropped in places where they’re out of place, and in a bunch of other ways glaciers modify landscapes. More recent evidence lets us probe further back in time. We’ve drilled into ice and rock to extract chemical evidence that tells us how Earth’s climate has changed over long timescales. This evidence tells us that Earth’s climate has varied significantly even without human interference. It’s been much warmer than the present day in the past, and it’s been much colder too. Clearly, we can’t expect the climate to remain fixed in future either. Let’s throw the idea that Earth’s climate has always been and should always be like the climate of today out the window!
3. Earth’s climate emerges from interactions of multiple phenomena.
What determines the climate on Earth? This is a long answer, which I can summarize in two words: it’s complicated! Why is the climate complicated? Because it emerges from the interactions of multiple processes that operate across a wide range of scales in space and time. The challenge of understanding something emergent is that we can’t apply the principle of reductionism to break it down into parts and study each piece separately. It’s the whole, or it’s nothing! We call the whole the climate system, which is an exemplar of a type of system that scientists call complex systems. You get the idea?
Just like it takes energy to get your ass out of bed in the morning, it takes energy to drive the climate system. The climate system runs on energy from the Sun in the form of electromagnetic radiation, which you can think of as a stream of particles called photons. Visible photons from the Sun hit the Earth and are absorbed as heat (a type of energy). The Earth emits infrared photons to maintain a balance; these photons are absorbed and re-radiated by Greenhouse gases in the atmosphere. Greenhouse gases are important here – Earth would have an average surface temperature of -18C without them!
The Earth is (roughly) a sphere, that orbits the Sun, and spins about an axis that is tilted relative to the ecliptic plane. All this spinning has some wobble to it. The spherical shape of our planet, the spinning, and the wobbling, mean that the amount of energy Earth receives from the Sun varies with space and time. We call the rate of flow of energy power, and the amount of solar power per unit area the solar irradiance. Because of Earth’s (roughly) spherical shape, solar irradiance is greater at the equator than poles. Much of the motion of air in the atmosphere and water in the oceans is a response to this differential irradiance, acting to redistribute heat from the equator to poles. The whole process is incredibly complex because of interactions between different pieces: the oceans and atmosphere ‘talk’ to each other by exchanging heat, momentum, and mass. The land and biosphere ‘talk’ to the system as well. A consequence of all this complexity is that the climate exhibits natural variability called ‘internal variability’.
4. Two types of evidence tell us that Earth’s climate is currently warming because of us.
We rely on two very different types of scientific evidence to understand climate change: data and models. Some people who haven’t studied these pieces of evidence are quite happy making uninformed statements about climate change. They may even do this quite loudly when they have a vested interest to do so. However, the evidence is very clear that (a) Earth’s global climate is getting warmer and (b) the warming is because of human greenhouse gas emissions.
Data – our direct measurements of Earth – are the most important type of evidence. Just as juries are presented with data as hard evidence for convicting criminals (e.g., fingerprints, DNA, ballistic analysis), data are the ‘smoking gun’ for climate change as well. There are multiple types of measurement that tell us about how the climate is changing. This includes measurements of the atmosphere, ocean, and cryosphere (ice) made on the ground and in space. All the measurements we have are incomplete. For example, measurements of the surface air temperature with thermometers don’t cover the oceans as completely as land, and don’t go back as far as we’d like for studying the climate (which varies over longer timescales than our measurements cover). Nevertheless, the surface thermometer measurements tell us that Earth’s average surface air temperature has gone up roughly 1.1C since the Industrial Revolution. This is corroborated by all the other types of data, such as measurements of sea ice extent, which has reduced by around 50% since we started measuring it in 1979. I won’t go into exhaustive detail here; the interested reader can do that themselves!
Models – computer simulations of the climate system – complement data in two ways. First, they provide important evidence that the warming trend is due to us (there are other ‘fingerprints’ in the data that tell us we’re the cause, but I won’t cover those here). We can apply all the known ‘forcings’ that can affect the climate (e.g., changes in solar irradiance, volcanic eruptions) in our computer simulations, but we can’t match the observed warming over the last 100 years without including our greenhouse gas emissions. The second thing that models can do is let us predict the future climate. We can run our models forward in time and inject greenhouse gases into the atmosphere at different rates to see what happens. This gives us some idea of what to expect under different ‘emissions scenarios’. If we had a way to collect data in the future, that would be better than models, but no-one has figured out how to build a time machine yet!
5. Most of the uncertainty in climate projections comes from what our society chooses to do.
The idea that we can take all the complexity of the climate system, simulate it on a computer, and still get accurate predictions might seem absurd. It’s certainly a challenging and imperfect endeavor. A famous example is clouds. When simulating the climate on a computer we need to ‘discretize’ it by breaking it down into discrete chunks called voxels. Think of how a digital photograph is broken down into pixels. Individual clouds are much smaller than the voxels in our climate models, so we need to ‘parameterize’ them. We need to somehow represent their effects in climate models, because clouds have a big effect, but we must do this without actually simulating clouds. This process of parameterization introduces uncertainty into our model simulations.
Scientists have done an inventory of the sources of uncertainty in predicting the future climate. The type of uncertainty that arises from the imperfections of the modeling process – things like parameterizations – is called ‘model spread’. Model spread grows with time: the longer we try to peer in the future with models, the greater the range of possible scenarios there is. There’s also uncertainty due to the natural internal variability of the climate system. This type of uncertainty is relatively small and doesn’t grow with time. Finally, there’s uncertainty over what our future emissions of greenhouse gases are. This is by far the biggest source of uncertainty. If we could look into the future and know the human population, the affluence of each person, and the amount of greenhouse gas emissions that affluence costs, we could predict our emissions. However, these things are all highly uncertain, leading to an uncertainty called ‘scenario spread’. Unknown things like future wars, future political elections, and future economic wealth all play a role in shaping this spread.
6. Implementing carbon pricing is key for averting severe climate change impacts; being politically active is your single most important contribution.
Climate models tell us that the remainder of the century is likely to see significant changes to our climate. These changes will cause suffering for people around the world in numerous ways. When I read about climate forecasts for 2100, I’m somewhat grateful that I almost certainly won’t be around then (I’d be a few years older than the current oldest living person). If we care about future people, what can we do to bring about a better future? Of course, we may end up living longer in future, and the effects don’t just start at 2100 but build up. In fact, we’re seeing some of them today. So, I’m thinking of my possible future self too!
Well, we could all try to reduce our individual carbon footprints. Become vegetarian. Fly less often. Live closer to work. Recycle. Etc. This might make you feel better, but I don’t realistically think it will make any difference! Global climate change cannot be solved by individual altruism. It’s too big a thing. Most people are either more interested in getting rich now, or they’re simply focused on staying alive! Thinking about future people is not a luxury that people living paycheck to paycheck have.
In the society we live in, money talks. Economists use the term ‘externality’ to refer to a cost that’s borne by society at large. There is a cost to society associated with greenhouse gas emissions that is not borne by the market. The solution is to make that cost explicit through instituting a price on carbon. This price could be implemented via a carbon tax, or via a cap and trade scheme. However it is implemented, pushing the real cost of burning fossil fuels onto buyers would encourage conservation and incentivize innovation in clean energy. In our free market economy, a carbon price is the only way to realistically effect change.
On an individual level, we may feel powerless to do much. Sure, you can try to reduce your carbon footprint, but the single biggest impact you can have is to be politically active. This starts with voting and goes from there!